Motor Control Flashcards
What is the difference btwn overload protection and overcurrent protection?
Overload = within normal path, damage normally thermal Overcurrent = outside normal path, damage thermal, magnetic and arcing
What is the difference btwn a schematic diagram and a wiring diagram?
Schematic shows only electrical relationships
Wiring shows electrical and physical relationship btwn components
What is electrical interlocking?
contacts placed in the control circuit to prevent energization of the forward and reverse contacts at the same time
List 2 types of thermal overload relays
Bi-metallic
Solder pot
Explain on delay
delay occurs upon energization
Explain off-delay
delay occurs upon de-energization
What is meant by the term jogging?
Energizing the motor for a very short interval
Stop buttons must always be connected in ____?
Series
Start buttons must be connected in _______ ?
Parallel
List three reasons why across the line starting is the most common method used for starting AC motors?
High starting T
Inexpensive
Simple
Why might across the line starting not always be suitable?
High starting current causing line disturbances
High starting torque damaging to some loads
List three types of reduced voltage starting methods?
Primary resistor or reactor
Autotransformer
Wye - Delta
Describe the term “open transition”
Motor momentarily disconnects from lines during transition
Describe the term “closed transition”
Motor continuously connected to lines during transition
What advantage is offered by closed transition versus open transition starting methods?
Less line disturbances with closed transition method
What advantage would reactors have over resistors for reduced voltage starting?
What disadvantage?
Reactor are more efficient
Poorer PF
What advantage is gained by the autotransformer starting method compared to the primary impedance method?
Less line current drawn for same torque
Can the wye-delta starting method be used for all 3 phase induction motors?
No, all leads must be brought out
What important consideration should be observed when selecting the OLR heaters for wye-delta starting method?
Must be sized with phase current
Sync Motor Control: Explain the purpose of the relay SCA
Shuts down circuit if motor does not accelerate within a certain amount of time or breakdown T is exceeded.
Protects the amortisseur windings from over heating
Sync Motor Control: Explain the purpose of the relay PFR
Senses voltage induced in rotor when out of step
Decides when to apply DC to the rotor for pull-in T to occur (92-97% of sync speed)
Sync Motor Control: Explain the purpose of the contactor FC
Controls contacts which connect DC and field resistor
Sync Motor Control: Explain the purpose of the reactor
Provides voltage drops for PFR to measure
Sync Motor Control: Explain the purpose of the discharge resistor
Protects against high induced voltages in field winding when DC is applied
Across the line voltage starters can result in extreme _______ and cause ______ ?
In rush currents
Brown outs
Reduced voltage starters are commonly employed for 3 phase SCIMS in the ____ to ___ range?
5hp to 200hp
Larger motors >200hp always use RVS
Primary resistance starter advantages and disadvantages?
Advantages: Simple, reduces voltage and current, smooth accel., OK for 3, 6, 9 & 12 lead motors
Disadvantages: Power consumption, inefficient, limited capacity